This invention relates generally to a fusing apparatus for fixing toner images on copy substrates, and more particularly to such an apparatus which affects fusing by the combined application of heat and pressure. This fusing apparatus is suitable for use in an electrostatographic recording machine such as, for example, a xerographic copier.
In a xerographic copier a light image of an original document to be reproduced is recorded in the form of a latent electrostatic image on a photosensitive member. The latent image is rendered visible by the application of a resin-based powder known as toner. The visual toner image is transferred electrostatically from the photosensitive member onto sheets of paper or other substrates. The toner image is then fixed or "fused", for example by applying heat and pressure, which causes the toner material to become soft and tacky whereby it is able to flow into the fibres or pores of the substrate or otherwise upon the surface thereof. Thereafter, as the toner material cools, it solidifies and is bonded firmly to the substrate. In the electrostatographic art generally the use of thermal energy and pressure for fixing toner images on to a substrate is well known.
It has long been recognised that one of the fastest and most positive methods of applying both heat and pressure for fusing the toner image to the substrate is by direct contact of the resin-based toner image with a hot surface, such as a heated roll which also applies pressure to the substrate. One approach is to pass the substrate with the toner image thereon between a pair of opposed roller members forming a nip, at least one of the rollers being internally heated. The actual temperature and pressure ranges will of course vary depending upon the softening range of the particular resin used in the toner. Typically, however, it will be necessary to heat the toner powder above 180.degree. C. Temperatures of 198.degree. C. or even higher are not uncommon in commercial fusers. Corresponding nip pressures are in the range of 690 to 1380 kNm.sup.-2.
A problem with this kind of fuser is that, as the toner becomes tacky, it can stick to the surface of the fuser roll which is undesirable because some of the toner on the fuser roll can then be transferred to subsequent substrates being fused and, moreover, those subsequent substrates will in their turn give rise to even more toner sticking to the fuser roll. This effect, known as "offset", clearly impairs copy quality. Furthermore, if the rollers are rotated when there is no substrate present in the nip therebetween, toner may also be transferred from the fuser roll to the backup roll so that when a substrate subsequently passes through the nip some of the toner may be transferred to the reverse side thereof.
An arrangement for minimising the problem of offset has been to provide a fuser roll with an outer surface or covering of, for example, polytetrafluorethylene, known by the trade name Teflon, to which a liquid release agent such as silicone oil is applied. The thickness of the Teflon is typically of the order of tens of microns and the thickness of the oil is less than 1 micron. Silicone based oils, for example polydimethylsiloxane, which possess a relatively low surface energy, have been found to be suitable for use in the heated fuser roll environment where Teflon constitutes the outer surface of the fuser roll. In practice, a thin layer of silicone oil is applied to the surface of the heated roll to form an interface between the roll surface and the toner images carried on the substrate. Thus, a low surface energy layer is presented to the toner as it passes through the fuser nip thereby preventing toner from offsetting to the fuser roll surface.
In attempts to improve the quality of the image fused by a heat roll fuser, such rolls have been provided with conformable surfaces comprising silicone rubber or Viton (trademark of E.I. duPont for a series of fluoroelastomers based on the copolymer of vinylidene fluoride and hexafluoropropylene). As in the case of the Teflon coated fuser roll, release fluids such as silicone based oils are applied to the surface of the silicone rubber or Viton to both minimise offsetting and to facilitate stripping. When the fuser system is one which provides for applying silicone oil to silicone rubber or Viton, a low viscosity silicone oil (i.e. in the order of 100 to 1000 centistokes) has most commonly been employed, although liquids of relatively high viscosity, for example 12,000 to 60,000 centistokes and higher, have also been used.
Various forms of applicators have been employed to supply the liquid release agent to the surface of the fuser roll, but perhaps the most straightforward technique is to use a wick, a part of which engages the surface of the roller and another part of which is immersed in a source of release agent contained in a reservoir. The wick is commonly made of a heat resistive nylon material called Nomex (trademark of E.I. duPont). U.S. Pat. No. 4,309,957 discloses a two-layer wick comprising a working surface material, such as felted or fibrous Teflon which contacts the fuser roll surface, and a backing material such as fibrous or felted Nomex to which the working surface material is needled. The release agent is conveyed from a supply reservoir to the backing layer by an annular conveying member.
U.S. Pat. No. 3,952,696 to Saupe discloses a fixing device which applies silicone oil to the heated fusing roll. This patent incorporates a heat insulating rotational support and a heat insulating web which extends partially around the roll (providing pressure engagement between the roll and copy). The web is securely fixed at one end, and is tensioned by a spring at the other end (see FIGS. 1 and 2). U.S. Pat. No. 4,182,263 to Naeser et al. discloses an applicator mechanism which applies fusing oil to a fixing roller. The oil is fed through a supply chamber into a first wick (felt) through openings in the chamber. This wick then transfers the oil to a second movable wick mounted upon supply/take-up reels. The second wick is urged into contact with the roller by two guide members.
U.S. Pat. No. 4,536,076 to Bickerstaff et al. discloses a liquid application apparatus consisting of two wick members - a supply wick and an applicator wick - and a reservoir above the wick secured to a manifold member. Liquid is delivered to the reservoir by a pump. Multiple orifices in the manifold permit the liquid to pass from the reservoir into the wick members. The applicator wick is secured in place by clips attached to its ends. The application apparatus is urged into contact with the fuser roll by spring members.
Another problem which is encountered in the prior art is that the fuser roll tends to become electrostatically charged during operation. This can be atributed to two main causes; the first being the triboelectric effect as the substrates, e.g. sheets of paper, are conveyed past the fuser roll in contact therewith, and the second being the transfer to the fuser roll of electrostatic charge which may already be present on the substrate from previous processing operations. One consequence of the fuser roll becoming charged is that the copy substrate will tend to stick to it giving rise to stripping problems. Perhaps more seriously, however, is the danger of electrostatic discharge. The charging effect is of course cumulative and the electrostatic charge on the fuser roll may build up to a level as high as 2 to 3 kV whereupon it will tend to discharge to the main frame of the machine which generally is maintained at earth potential. The result is an electrical noise spike which can be sufficiently severe as to adversly affect the electrical control system of the xerographic machine and may cause the machine to reset itself to the default condition (i.e. in readiness to start a new job) while a copy run is still in progress, or it may even lead to total shutdown or failure of the whole machine.
Attempts have been made to overcome this problem. For example the fuser roll has been mounted in electrically conductive bearings in the frame of the copier which is earthed thus providing a leakage path to earth. This solution is not entirely satisfactory however because the conductive bearings also provide an undesirable thermal path to the machine frame which reduces the efficiency of the heated roll, and furthermore does not effectively remove the electrostatic charge at the roll surface.
Japanese Patent Document No. 0053874 to Sakurai discloses a fixation device which includes a conductive needle like member in contact with either a heated fixing roll or a back-up roll. The conductive member prevents the build-up of electrostatic charge on the roll. The member may be composed of a combination of metallic and carbon fibers.
U.S. Pat. No. 3,288,175 to Valko discloses a textile material which conducts electricity, but this is not a wick material. The material is composed of non-metallic threads in a continuous system of metallic fibers (up to 10 percent). The material conducts electricity and has the aesthetic properties of a non-metallic textile.